Supplementary Information
1. Theory When an electric potential is applied between a metal-electrolyte interface, charges are redistributed and accumulate at the solid-liquid interface, forming an electrical double layer (EDL), which changes the surface energy of the solid-liquid interface [1]. Repulsion of like charges at the solid-liquid interface reduces the work required to expand the surface area, leading to a decrease in the associated surface tension and induces a change in the contact angle [1]. This phenomenon is called electrowetting [2]. The relationship between contact angle and applied electric potential is described using the Lippmann-Young equation, which can be derived using different approaches – the classical thermodynamic approach, the energy minimization approach, and the electromechanical approach. Here we present the classical thermodynamic approach in detail. The other two approaches can be found elsewhere [3].
a. The classical Thermodynamic Approach Consider a droplet of conductive liquid (electrolyte) placed on a smooth metal surface. The metal surface is in direct contact with the electrolyte, and a small potential difference is applied such that no current flows through the liquid and no Faradaic reactions take place on the metal surface [3]. Due to the application of an electric field, an electrical double layer (EDL) is formed in the liquid which is in contact with the metal surface [3]. The mathematical relationship between the surface charge distribution, surface tension and electric potential can be derived from Gibb’s interfacial thermodynamics [3], which yields: 𝑑𝛾 −𝜌 = (1) 𝑑𝑉